A spectrometer is a device which receives a light signal as an input and produces as an output a light signal which is spread out in space according to the different wavelength components, or colors, of the input light signal. A detector attached to the spectrometer analyzes the output signal, called the spectrum, to quantify the amount of each wavelength component which is present in the input signal. One specific type of spectrometer is known as an Offner spectrometer which can be used to produce images of a remote object over a contiguous range of narrow spectral bands. This type of imaging is known as hyperspectral imaging and has recently emerged as an important part of the military/aerospace solution to airborne and spaceborne reconnaissance and remote sensing. Basically, the hyperspectral imaging system utilizes an Offner spectrometer and an advanced data processing technology to produce imagery with embedded spectral signature data. This signature data is useful in a wide-variety of applications such as target designation/recognition, missile plume identification and mine detection (for example). In addition, the hyperspectral imaging system can be used in a wide-variety of commercial applications such as cancer detection, environmental monitoring, agricultural monitoring and mineral exploration. An exemplary conventional hyperspectral imaging system which incorporates an Offner spectrometer is discussed below with respect to FIGS. 1A-1B (PRIOR ART).
Referring to FIGS. 1A-1B (PRIOR ART), there are shown two perspective views of an exemplary conventional hyperspectral imaging system 100 which incorporates an Offner spectrometer 102. The hyperspectral imaging system 100 includes a first housing 104 which is positioned next to and attached to a second housing 106 (see FIG. 1A). The first housing 104 encloses and protects a single fore optic 108, a slit 110 (with a single opening 111), and a 2-dimensional detector 112. The second housing 106 encloses and protects the Offner spectrometer 102 (see FIG. 1B). In this example, the Offner spectrometer 102 is a one-to-one optical relay which includes an entrance opening 114 (can be same as or adjacent to slit's opening 111), a first mirror 116, a diffraction grating 118, a second mirror 120 and an exit opening 121 (positioned next to the 2-dimensional detector 112). It should be appreciated that for clarity the description provided about the conventional hyperspectral imaging system 100 omits certain details and components which are well known in the industry and are not necessary to explain and understand the present invention.
The conventional hyperspectral imaging system 100 operates to produce images of a remote object 105 over a contiguous range of narrow spectral bands when the fore optic 108 receives a beam 107 from the remote object 105 and directs the beam 107 to the slit's single opening 111 which outputs a trimmed beam 122 (slice of the image) to the Offner spectrometer 102 which diffracts the trimmed beam 122 and forwards the diffracted beam 124 to the detector 112 (see FIGS. 1A and 1B). In particular, the slit's single opening 111 outputs the trimmed beam 122 which passes through the entrance opening 114 (if present) and is received at the first mirror 116 (spherical mirror 116) which reflects the trimmed beam 122 towards the diffraction grating 118. The diffraction grating 118 receives the trimmed beam 122 and diffracts and reflects the diffracted beam 124 to the second mirror 120 (spherical mirror 120). The second mirror 120 receives the diffracted beam 124 and reflects the diffracted beam 124 through the exit opening 121 to the detector 112. The detector 112 (e.g., two dimensional focal plane array (FPA) 112) receives and processes the diffracted beam 124 which passed through the spectrometer's exit opening 121.
This type of hyperspectral imaging system 100 generally works well in most applications however in the short wave infrared (SWIR) wavelength band (0.75-2.5 μm) and the long-wavelength infrared (LWIR) wavelength band (7-15 μm) the current commercially available detector 112 has a limited number of pixels which can be used to image when compared to the commercially available detectors associated with the visible wavelength band. In particular, the current commercially available detector 112 has a limited number of pixels that can be used to image the remote object 105 in a two dimensional focal plane which is composed of a spatial direction and a spectral direction. Thus, to improve the spatial field coverage at a particular resolution, multiple conventional hyperspectral imaging systems 100a, 100b . . . 100n are currently located side-by-side such that the “linear field of view” of each conventional hyperspectral imaging system 100a, 100b . . . 100n are aligned end-to-end with one another to image the remote object 105 (not shown) at a particular resolution as shown in FIG. 2 (PRIOR ART). This solution is prohibitive for many applications including the SWIR and LWIR applications due to the space, weight, power constraints, and costs of the multiple detectors (which are very expensive), coolers, spectrometers etc.